Digital SLR lenses typically are quicker to focus and come with a variety of creative features, including built-in optical image stabilization for steadying shaky shots and wide apertures for shooting in low light. The problem is that many of the best lenses are expensive—not to mention so big and heavy that you wouldn't want to carry one around all day. This SLR lens guide can help take some of the confusion out of shopping.
How lenses on a DSLR work
An interchangeable lens is attached to a digital SLR via a locking mechanism called a "mount." Once it's mounted, the lens provides you with a specific "focal length." (Technically, that focal length is the distance between the lens' glass elements and the camera's sensor, which captures the image). The greater a lens' focal length, the closer it brings the subject you are shooting. A given lens is usually designed to work with just a single brand of camera. For example, a Nikon lens will fit a Nikon DSLR, but not a Canon. See the Brands section for more information.
A lens is like a tube filled with glass lenses that funnel light onto a digital sensor. The physical size of the lens, and the amount and shape of the glass will affect its focal length. A standard or "normal" lens, with a fixed focal length of 50 mm, has a horizontal angle of view that is about the same as what the human eye perceives. A longer lens, such as a telephoto, will bring you closer to your subject while a shorter, rounder, wide-angle lens will give you a broader perspective.
Some lenses, called zooms, provide a range of focal lengths that you can control as you shoot. Some of the most typical zoom lenses used with consumer DSLRs range from 18 to 55 mm to 24 to 85 mm. For shooting subjects at greater distances, there are telephoto zoom lenses, with ranges such as 70 to 200 mm. For shooting landscapes or other wide subjects, there are wide-angle zooms, with a range such as 12 to 24 mm. For the greatest versatility, there are ultra zooms, with ranges as great as 28 to 135 mm or 18 to 200 mm.
Because the image sensors on digital cameras are usually smaller than a frame of traditional 35-mm film, when a lens is used with a DSRL, you need to account for that difference by figuring its effective focal length. For example, a 50-mm lens on a Canon Digital Rebel would have an effective (or "equivalent") focal length of 80-mm lens when taking into account the camera's 1.6x magnification. Such magnification factors vary from one brand of camera to another, and sometimes even among models within the same brand.
Some higher-end digital SLRs use so-called "full-frame" sensors, which are about equal in size to a frame of 35-mm film. With such cameras, there is no lens magnification, or equivalent focal length. In short, a 50-mm lens remains a 50-mm lens on a full-frame sensor SLR such as the Canon 5D Mark II or Nikon D700.
Kit lens vs. interchangeable lens
If you just purchased a digital SLR, you might be wondering why you'd need to buy a lens if your camera came with one. It's true that most consumer digital SLRs are sold with a "kit" lens—typically a short standard zoom with a focal range of around 18 to 55 mm (28-to-85-mm digital equivalent)—which is cost effective, but not necessarily of the highest quality.The problem with such lenses is that they limit what you can do with them creatively because they're usually not long enough to reach distant subjects such as wildlife and they're not wide enough to capture a broad perspective such as when photographing landscapes. Kit lenses are also not ideal for portraits because their focal lengths are shorter than what's preferred for portraits and their apertures aren't suited for blurring the background, which helps to draw attention to the person you're shooting. Kit lenses aren't as rugged or as sharp as more-advanced lenses because they're typically made of plastic rather than metal and glass. To see how some inexpensive "kit" style lenses compare with expensive ones, see our Ratings [link to lens ratings] of SLR lenses (for subscribers).
Even if you can afford one, a higher-priced, higher-quality lens might not always be your best choice. The most important consideration in choosing a lens is that its aperture, focal length, and other features match the type of shooting you intend to do.
For a good-quality standard zoom lens, expect to pay at least $300. If you want one with a wider aperture—i.e. a smaller f/stop number such as f/2.8, which lets in more light—you could pay two to three times that.A quality wide-angle or telephoto zoom lens can range from expensive to extremely expensive, so expect to pay at least $600 for a decent wide or long zoom. Any lens with an aperture of f/2.8 or faster sells for $1,000 or more.
Because they don't offer the versatility of a zoom, fixed focal length lenses are much less expensive even when they offer a fast aperture well suited for taking portraits. Most point-and-shoot cameras can't replicate this effect because their sensors are very small—the bigger the sensor, the shallower the depth of field. A 50-mm lens with an f/1.4 aperture will sell for about $300. A 50-mm lens with an f/1.8 aperture sells for less than $100.
In addition to costing less than a zoom, a fixed-focal-length lens is usually sharper and brighter, with better contrast, and suffers from fewer defects, mainly because it's made with fewer glass elements.
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